1. Field of the Invention
The present invention relates to a Cu—Zn—Si based alloy having excellent castability, mechanical properties (strength, ductility etc.), corrosion resistance, wear resistance, machinability and the like.
2. Description of the Related Art
It has been known that copper alloys are improved in yield strength by grain refinement like ordinary metal materials, and that in accordance with the Hall-Petch law the copper alloys are improved in strength in proportion to the inverse of the square root of the grain diameter.
And the copper alloys are generally subjected to two basic types of grain refinement as follows: (A) when the copper alloys are melted and solidified, (B) when the copper alloys (ingots such as slabs, castings such as die castings, melted castings etc.) after melt-solidification are subjected to either deforming such as rolling or heating, and the resultant stored energy such as distorted energy acts as a driving force. In either case of (A) or (B), Zirconium (Zr) is known as an element that effectively affects the grain refinement.
However, in the case of (A), since the grain refinement effect of Zr in the step of melt-solidification is considerably influenced by other elements and their contents, a desired level of grain refinement is not achieved. For this reason, generally, the technique of (B) has been widely used, wherein the grain refinement is facilitated by performing heat treatment on the ingots, castings and so forth after melt-solidification, and then endowing distortion again.
According to teachings of Japanese Examined Patent Application Publication No. 38-20467, a copper alloy containing Zr, P and Ni is subjected to melting treatment, cold working at a rate of 75%, and examination of its average grain diameter, in which the average grain diameter is decreased in proportion to increase of a content of Zr, for example, 280 μm when not containing Zr, 170 μm (Zr content: 0.05 mass %), 50 μm (Zr content: 0.13 mass %), 29 μm (Zr content: 0.22 mass %) and 6 μm (Zr content: 0.89 mass %). In this document, it is proposed to contain 0.05 to 0.3 mass % Zr in order to avoid an adverse effect caused by excessive content of Zr.
Further, it is disclosed in Japanese Unexamined Patent Application Publication No. 2004-100041 that when a copper alloy to which 0.15 to 0.5 mass % Zr is added is subjected to casting, melting treatment, and deformation processing for distortion addition, its average grain diameter is refined to a level of about 20 μm or less.
However, as in the technique of (B), these treatment and working after casting for refining the grain diameter result in increased costs. Further, some castings can not be subjected to the deformation processing for distortion addition due to their shapes. As such, the grains are preferably refined by the technique of (A) when the copper alloy is melted and solidified. However, in the case of the technique of (A), as set forth above, Zr is greatly influenced by other elements and their contents in the step of melt-solidification. Hence, although the content of Zr is increased, the grain refinement corresponding to the increase is not necessarily achieved. Further, Zr has very strong affinity for oxygen. Accordingly, when being melted and added in the atmosphere, Zr easily forms an oxide and is very low in yield. As such, although a very small quantity of Zr is contained in products after casting, it is required to charge a considerable quantity of raw material in the step of casting. Meanwhile, when being too much produced during melting, the oxide is easily entangled when casting, there is a chance to generate casting defects. In order to avoid production of the oxide, the melting and casting may be carried out under a vacuum or inert gas atmosphere, which causes increase of costs. In addition, because Zr is an expensive element, its addition amount is preferably restrained to be as small as possible from the economic point of view.
For this reason, there is required a copper alloy having the content of Zr as small as possible and simultaneously the average grain diameter refined in the following step after melt-solidification of the casting process.
Further, in the case of the Cu—Zn—Si based alloy, Si serves to improve mechanical property etc., but during melt-solidification, has problems that it is easy to generate a crack or porosity, that a shrinkage cavity is great, and that it is easy to generate casting defects such as a blow hole. The main reason is because as a content of Si increases, a solidification temperature range (a difference between a liquidus temperature and a solidus temperature) becomes wide, and a thermal conductivity is also deteriorated. Further, taking a view of a solidification structure of a conventional Cu—Zn—Si based alloy, a dendrite is generated in a tree-like branching pattern. Arms of the dendrite make it difficult to discharge generated air bubbles into the air, which is responsible for residual of blow holes, and local generation of great shrinkage cavity.
The present invention provides a Cu—Zn—Si based alloy capable of significantly improving copper alloy properties such as castability, various mechanical properties, corrosion resistance, machinability, workability etc. by means of refinement of grains, and simultaneously a method of fabricating the same.